The present invention relates to an improvement in a flow rate control valving apparatus suitably used in a vehicular power steering system and the like.
Since an oil pump which serves as a hydraulic pressure generating source in, e.g., a hydraulic power steering system is driven by an engine of a vehicle, a discharge amount of a pressurized oil from the pump is increased/decreased in proportion to the engine speed. Therefore, such a pump is required to have a capacity capable of providing a sufficient flow rate not interfering with an operation of the power steering system as a fluid system even when a pump discharge amount is small. However, a pump having this capacity provides an unnecessarily high flow rate in a high speed range of an engine to pose various problems in individual parts. In conventional systems, therefore, a flow rate control valving apparatus for controlling a supply flow rate to be constant is generally disposed in an oil pressure passage between the pump and the power steering as a fluid system to return an unnecessary pressurized oil to the tank.
As a flow rate control valving apparatus of this type, an apparatus having an arrangement as shown in FIG. 4 is conventionally known. This apparatus will be briefly described below. A flow rate control valving apparatus generally denoted by reference numeral 1 comprises a valve housing 2 formed or provided integrally with a pump body, a valve bore 3 having an end open in a portion of the housing 2, a pressurized oil supply passage 4 and a pressurized oil return passage 5, open in a central portion in the axial direction of the valve bore 3 with a predetermined interval therebetween, for supplying a pressurized oil from a pump P and returning the oil to a tank T, respectively, a valve spool 6, slidably held in the valve bore 3, for selectively communicating or closing the two passages 4 and 5, a set spring 7 for biasing the spool 6 toward the open end side of the valve bore 3 to normally close the passages 4 and 5, and a plug connector 9 threadably engaged with the open end of the valve bore 3 and internally having a pressurized oil supply passage 8 for communicating with the oil pressure supply passage 4 from the pump to supply the pressurized oil to a power steering PS. A small-diameter opening 10 is formed in the connector 9 at a portion close to the pressurized oil supply passage 4, and an adjusting rod portion 11 extending through the small-diameter opening 10 and having a large-diameter head portion at its distal end is integrally provided coaxially with the spool 6, thereby constituting a metering orifice 12 for moving the spool 6 against the biasing force of the spring 7 in accordance with a pressurized oil supply flow rate from the pump P to selectively allow the passage 4 to communicate with the passage 5 to the tank T. That is, the pressure on the downstream side of the orifice 12 is supplied to a low-pressure chamber 15, formed at the end portion on the spring 7 side of the spool 6, by a passage bore 13a formed in the radial direction of the connector 9 and a passage bore 14 formed in the valve housing 2 through an annular recess 13b connected to the passage bore 13a.
The pressure on the upstream side of the orifice 12, on the other hand, is supplied from the supply passage 4 to a high-pressure chamber 16 formed between the end face from which the adjusting rod portion 11 of the spool 6 projects in the valve bore 3 and the small-diameter opening 10 for forming the orifice 12. As a result, the spool 6 is moved in either direction by a differential pressure generated before or after the orifice 12 in accordance with the supply flow rate of the pressurized oil to adjust an amount to be returned to the tank T, so that the supply flow rate to the supply passage 8 is maintained constant.
Reference numeral 17 denotes a stopper cylinder, projecting from the inner end of the connector 9 to form the high-pressure chamber 16, for regulating a moving amount of the spool 6 toward the high pressure side by stopping it at a predetermined position. Note that reference numeral 17a denotes a bore portion for guiding the pressurized oil into the stopper cylinder 17; and 6a, an annular recess formed adjacent to a land portion 6b of the spool 6 for closing the passages 4 and 5 and connected to the return passage 5. Other arrangements or operations including these parts except for those described above are conventionally known and a detailed description thereof will be omitted.
In the above flow rate control valving apparatus, since the metering orifice 12 must be provided in the pressurized oil supply passage in order to move the spool 6 to execute flow rate control, a pressure loss derived from the presence of the orifice 12 cannot be avoided. That is, assuming that the sectional area of the spool 6 is A, the pressures before and after the metering orifice 12 are P.sub.1 and P.sub.2, respectively, a spring constant is k, a deflection amount of the spring 7 is x, and a deflection amount upon setting is .delta., the following equation is given: EQU P.sub.1 A=P.sub.2 A+k(x+.delta.)
Therefore, a pressure difference represented by the following equation is necessary to move the spool 6: EQU P.sub.1 -P.sub.2 =k/A.multidot.(x+.delta.)
Since the individual parts such as the passages of the orifice 12 must be designed in accordance with the above conditions, generation of the pressure loss cannot be avoided.
In recent years, however, a strong demand has arisen for energy saving, and a valve arrangement capable of minimizing the pressure loss caused by the metering orifice 12 described above has been desired accordingly.
In addition, a flow rate control valving apparatus of this type is required to realize a simple arrangement, a light weight, and a low manufacturing cost, and a certain countermeasure allowing easy processing of the individual parts is also necessary. Therefore, these conditions must be taken into consideration.